Orthodontic spring



FIG. 1 is a top, front, left side perspective view showing my new design;

FIG. 2 is a left side elevation view;

FIG. 3 is a front elevation view;

FIG. 4 is a right side elevation view;

FIG. 5 is a rear elevation view;

FIG. 6 is a top plan view; and,

FIG. 7 is a bottom plan view. 

The ornamental design for an orthodontic spring, as shown and described. 